Don't save non-errors in the memstore iterator

As per the following link, io.EOF is returned to signal "no more items".
Thus, we don't treat this as an error, and don't save it.

https://godoc.org/github.com/cznic/b#Enumerator

graph/memstore/iterator.go

@@ -15,6 +15,7 @@
 package memstore
 
 import (
+	"io"
 	"math"
 
 	"github.com/google/cayley/graph"
@@ -121,7 +122,9 @@ func (it *Iterator) Next() bool {
 	}
 	result, _, err := it.iter.Next()
 	if err != nil {
+		if err != io.EOF {
 			it.err = err
+		}
 		return graph.NextLogOut(it, nil, false)
 	}
 	if !it.checkValid(result) {